This invention relates to a fuel control system for a gaseous fueled engine, and more particularly to an improved feedback fuel control system for an engine.
It has been proposed to operate internal combustion engines on gaseous fuels such as LPG, LNG, CNG (liquid petroleum gas, liquid natural gas, or natural gas). These fuels have been found to promote longer engine life and also offer the opportunity of conserving natural resources and providing better exhaust emission control.
Normally the fuel is supplied to a charge former where it is metered and mixed with air to form a charge that is delivered to the engine through its induction system. It has been proposed to employ a so-called air valve or constant depression type of carburetor for mixing the fuel with the incoming air. This type of charge former has been found to provide better fuel control with these particular types of gaseous fuels. The main fuel adjustment is set with these carburetors by a movable valve element which controls the effective air flow area. Frequently this element is a sliding piston, although pivoted valves may also be employed. A metering rod is connected to the movable element and cooperates with a jet for controlling the size of the fuel flow orifice. This controls the amount of fuel flowing to the engine and hence the mixture strength.
With this type of charge former or with more conventional fixed venturi types of charge formers, the amount of fuel delivered can be fine tuned by mixing a proportion of air with the fuel that is delivered to the metering jet. This air mixture can be controlled from a sensor such as an oxygen sensor to provide very effective feedback control. Such an arrangement is shown and described in U.S. Pat. No. 5,337,722, entitled "Fuel Control and Feed System for Gas Fueled Engine," issued Aug. 16, 1994, and assigned to the assignee hereof.
As disclosed in that patent, the air bleed control is operated by a stepper motor, and the number of steps is varied in response to the output signal from the sensor so as to maintain the air-fuel ratio at a stoichiometric value. It has been found, however, that improvements are possible with this type of arrangement.
It is, therefore, a principal object of this invention to provide an improved fuel control system and method of controlling fuel for an engine that will be more responsive to the actual needs in its feedback control operation.
With the previously proposed system, when it has been determined that the mixture is not a desired or stoichiometric mixture, the stepper motor is moved to a position that has been memorized from previous experiences as providing a stoichiometric ratio. This step position is basically independent of the engine running condition.
It has been found, however, that the amount of control required varies with engine running characteristics, and specifically engine load. This may be understood partially by reference to FIG. 1, which is a curve showing engine output or load in relation to speed at various stepper motor step positions. The step positions shown are with a control where the stepper motor moves through 200 steps from a fully closed zero position to a fully opened position.
In the fully closed position there is no dilution of the fuel that is supplied. The practical effect of this is that the actual step number should be lower during high load operations and may be increased during low load operations to maintain stoichiometric air-fuel mixture. Thus, the step adjust, which was constant with the prior art type of constructions, must be varied in relation to load in order to attain the target stoichiometric air-fuel mixture.
It is, therefore, a still further object of this invention to provide a feedback control for the fuel supply of an internal combustion engine wherein the feedback control is varied in relation to engine load.
This type of system operates the stepper motor to control a flow orifice, and that stepper motor is moved through a number of steps based upon a feedback coefficient (FK), which is multiplied by the step number (STP) in accordance with the following relationship: EQU STP=FK.multidot.STP
As the engine conditions change, the feedback control also is changed over a time period in accordance with the relationship dFK/dt.
Another problem with the prior art type of operations may be understood by reference to FIG. 2, which shows actual fuel flow in relation to intake air flow at three positions: stepper motor fully closed (STP=0), no fuel dilution; stepper motor half opened (STP=100), partial fuel dilution; and stepper motor fully opened (STP=200), maximum fuel dilution. It will be seen in the range of low air volume that the fuel flow does not vary linearly with air flow. That is, in order for fuel to flow into the venturi section formed by the variable throat, the intake air volume must be above a predetermined pressure so as to generate the required negative pressure for the venturi to function. This may be in the range of -4 mm of mercury gage pressure. Therefore, with the prior art structures it is difficult to maintain the target or stoichiometric air-fuel ratio in the range where the intake air volume is low, such as the range shown in FIG. 2 as AA.
It is, therefore, a still further object of this invention to provide an improved feedback control for the fuel flow of an engine that will provide the desired amount of fuel under all running conditions.
The prior art systems also made their feedback adjustment during a fixed time intervals regardless of the running condition of the engine. It has been found that this may not offer the maximum utilization of the catalyst, particularly when a three-way catalyst is employed. Such three-way catalysts operate to oxidize carbon monoxide (CO) and hydrocarbons (HC) and reduce oxides of nitrogen (NO.sub.x). FIG. 3 is a graphical view showing the cleaning efficiency on these three types of exhaust gas constituents in relation to the frequency of application of the feedback control. It will be seen from these curves that the feedback frequency also is highly important in controlling the treatment of exhaust gases.
It is, therefore, a still further object of this invention to provide an improved fuel control system for an engine embodying feedback control and wherein the intervals of feedback frequency are varied in response to the engine running conditions.
FIG. 4 is a graphical view showing how the feedback frequency should be varied in response to engine running conditions such as intake air volume to achieve maximum efficiency of the catalyst. It is, therefore, a still further object of the invention to provide a feedback control system for the fuel supply of an engine wherein the frequency of feedback control is increased as the air flow to the engine increases.